Current cellular services are provided around a frequency band of 800 MHz. These systems are known as "high-tier" systems. "High-tier" refers to a system that is characterized by high base stations, high transmission power, and several miles of coverage, and which can support wireless subscribers (for example, cellular telephones) which are traveling at relatively high speed, such as approximately 30 to 40 miles per hours or more.
One such high-tier system is the Global System for Mobile Communication (GSM) which is deployed in many areas of the world including parts of North America, Europe and Asia. GSM supports the subscriber units in a service area by managing location and identification information for each subscriber unit, transmitting system information and incoming calls to subscriber units, and receiving transmissions from subscriber units. GSM also interfaces the subscriber units with the local public switched telephone network (PSTN).
A cellular base station in a GSM system is typically a large and expensive structure. A high-tier base station may be hundreds of feet high and service a ten mile radius. In addition to their size and expense, traditional cellular communications are frequently of very low voice quality (8 kilobytes/second to 13 kilobytes/second) compared to the voice quality of low-tier wireless access technologies (32 kilobytes/second) when the subscriber unit is used indoors.
Traditional high-tier cellular systems are further limited in the amount of traffic they can carry. When enough subscribers attempt to use the system at the same time, high-tier systems become congested and subscribers' calls are blocked. "Cell splitting" is a known method of increasing the capacity of high-tier systems, but when enough demand is experienced, cell splitting ceases to be effective.
In addition to these systems, "low-tier" radio standards have been developed for wireless communication. One such low-tier system is the Personal Access Communications System (PACS), a radio standard for wireless communication that operates in the 1850-1990 Mhz frequency band.
PACS has several advantages over the known high-tier systems, such as small and inexpensive base stations and a much lower transmission power requirement. The development of PACS is described in the following articles which are incorporated herein by reference: D. C. Cox, Universal Digital Portable Radio Communications, Proceedings of the IEEE, Vol. 75, Apr. 1987, pp. 436-477; D. C. Cox, A Radio System Proposal for Widespread Low-Power Tetherless Communications, IEEE Trans. on Comm., February 1991, pp. 324-335; V. K. Varma et al., A Flexible Low-Delay TDMA Frame Structure, IEEE ICC '91, Denver, Colo., Jun. 23-26, 1991; American National Standards Institute J-STD-014, Personal Access Communication System Air Interface Standard, 1995 (hereinafter "ANSI J-STD-014").
The advantages of PACS stem from the small size of the base stations used. Known as radio ports (RPs), these base stations service a relatively small area (path lengths up to a few kilometers). Being both small and relatively inexpensive, RPs can be widely deployed on utility poles, on buildings, in tunnels, indoors, and so forth, to provide more comprehensive support for wireless access services. Additionally, RPs have relatively small power needs compared to high tier base stations. An RP can be line or battery powered.
PACS also has operational advantages. Its narrow band transmission format creates a relatively large number of frequency channels in the 10 MHz sub-bands from which an RP can choose. This allows PACS to have higher frequency reuse factors which allows for more efficient utilization of the available radio frequency band. Additionally, because most of the circuitry is located in radio port controller units (RPCUs) that service a number of RPs, upgrading the system can be accomplished without visiting each RP.
PACS, however, is not without limitations. As noted, PACS is a low-tier system and can only support subscriber units that are moving at relatively low speeds. Because of the small service areas supported by each RP, subscriber units traveling in excess of 30 to 40 miles per hour will require frequent hand-off or handover from one RP or base station coverage area to another. This is one reason that PACS is most cost effective in areas with a high density of slow moving (for example, pedestrian) subscribers.
PACS was originally developed under the assumption that the radio port network would be interfaced with the public switched telephone network (PSTN), specifically, to an ISDN switch. To support PACS, a PSTN must perform mobility management functions which are similar to those performed by GSM. Specifically, a PACS-supporting PSTN must track the location within the service area of each subscriber unit and manage the routing of calls.
In order to support the above-mentioned wireless access mobility management functions using PACS, a PSTN with Advanced Intelligent Network (AIN) capabilities will be required. However, PSTNs in many areas, especially in developing nations, are not equipped with AIN capabilities. Therefore, an alternative architecture is desired which would allow the rapid deployment of PACS so as to provide wireless services in such areas.
As described in detail below, supporting PACS using a GSM system will provide this alternative architecture.